⚡ Optimize Gem Addition Logic

[google-labs-jules]

💡 What:

• Refactored GemExecutor.addGems to use a new addGems method in MySQLManager.
• Implemented addGems in MySQLManager using INSERT ... ON DUPLICATE KEY UPDATE to handle both new and existing users in a single query.
• Added unit tests GemExecutorTest and MySQLManagerTest to verify the fix and prevent regressions.
• Updated pom.xml to include test dependencies (junit, mockito-inline, byte-buddy) and fix PlaceholderAPI resolution.

🎯 Why:

• The previous implementation performed up to 5 redundant database queries (2 Selects, 1 Insert, 2 Updates) per operation.
• This caused unnecessary latency and load on the database.
• The new implementation performs exactly 1 database query, significantly improving performance and atomicity.

📊 Measured Improvement:

• Verified via GemExecutorTest that database interactions were reduced from ~5 calls to 1 call.
• Verified via MySQLManagerTest that the generated SQL is correct and efficient.

---

PR created automatically by Jules for task 1554755171763117530 started by @acsoto

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.